Spin selection in atomic-level chiral metal oxide for photocatalysis

The spin degree of freedom is an important and intrinsic parameter in boosting carrier dynamics and surface reaction kinetics of photocatalysis. Here we show that chiral structure in ZnO can induce spin selectivity effect to promote photocatalytic performance. The ZnO crystals synthesized using chiral methionine molecules as symmetry-breaking agents show hierarchical chirality. Magnetic circular dichroism spectroscopic and magnetic conductive-probe atomic force microscopic measurements demonstrate that chiral structure acts as spin filters and induces spin polarization in photoinduced carriers. The polarized carriers not only possess the prolonged carrier lifetime, but also increase the triplet species instead of singlet byproducts during reaction. Accordingly, the left- and right-hand chiral ZnO exhibit 2.0- and 1.9-times higher activity in photocatalytic O2 production and 2.5- and 2.0-times higher activities in contaminant photodegradation, respectively, compared with achiral ZnO. This work provides a feasible strategy to manipulate the spin properties in metal oxides for electron spin-related redox catalysis.

The work described the preparation and characterization of chiral ZnO layers. Beyond serving as spin filter that can be used in reactions that involve triplet oxygen, the authors found that the chirality of the film prolongs the lifetime of the electron-hole following photoexcitation. The study is very interesting and convincing. However, two issues must be improved before this manuscript can be accepted for publication. 1. There are no error-bars in the data presented and not estimation of the accuracy of the result is given. In addition it is not clear how many samples were prepared and what is the variation in their properties. 2. In the introduction it is not clear what of the properties of chiral systems mentioned were really observed and what properties one can assume to exist but they were not observed so far. A clear distinguishing between the two class must be made. Once the authors correct that two points the manuscript should be published in Nat. Comm.

Reviewer #3 (Remarks to the Author):
Most researchers regulated the spin degree of freedom in transition metals directly, which depends greatly on fine-tuning the coordinated structure of metals or producing spinpolarized carriers by using spin filters. In this study, the authors fabricated the hierarchical chirality of ZnO photocatalyst as the prototype by asymmetric coordination between the chiral center of the methionine molecules and Zn2+ ions. The chiral structures acted as the spin selection filter and induced spin polarization during the photoinduced carrier separation. Interestingly, the photoexcited carrier lifetime was extended by suppressing the carrier recombination, and spin-dependent kinetics of water oxidation reaction and pollutants mineralization reaction was significantly promoted by inhibiting the singlet H2O2 formation, and as a result, the left-and right-hand chiral ZnO showed greatly enhanced photocatalytic activity for the O2 production and photodegradation of pollutants. Overall, it is an interesting study, and the conclusions are mostly supported by the data, which may merit publication after fully addressing the following issues. 1) Page number is missing in the manuscript. 2) Abstract: "respectively" should be added immediately after "photodegradation" 3) Introduction: some closely-related publications need to be cited to enhance the research background, e..g, magnetic-field-promoted photocatalytic overall water-splitting (Energy Environ. Sci., 2022, 15, 265-277; Chem Catal. 2022, 2, 221-241) 4) The authors claimed that chiral ZnO was fabricated by adopting the amino acid-induced self-assembly method reported in the literature (ref. 40). Did the authors modify the method? 5) Figure S2, caption: "is precursor" should be replaced by "are the precursor" 6) Figure S7h: the crystal lattice fringe should be 2.64 nm / 9 = 0.2933 nm rather than ca. 0.26 nm. Please double-check. 7) Figure S8b-b2: why a different scale (250 nm) was used for these figures? It would be better to use the same scale as shown in S8a and S8c (i.e., 200 nm) 8) Figure S9: pore size distribution should be also provided in this figure. 9) Are there any mesopores or hierarchical porosity in the as-synthesized materials? 10) The authors mentioned that "all of the fabricated ZnO samples show a similar surface area ( Figure S9, SI)," But they did not tell what the pore size is for the as-synthesized porous materials. In literature, the presence of mesopores (or macropores) favors multilight scattering/reflection, resulting in enhanced harvesting of the exciting light and thus improved photocatalytic activity (CrystEngCommun 2022, 24, 6498-6504; J. Clean. Prod. 2021, 328, 129745; Environ. Chem. Lett. 2021, 19, 3573-3582). The authors are suggested to consider similar effects from the mesopores (or macropores) if applicable. 11) The authors stated that "the high-resolution O 1s spectra in Figure S10b (SI) can be indexed into two distinctive binding energy peaks at 529.9 and 531.8 eV, which belong to the lattice oxygen and adsorbed oxygen/hydroxyl radicals on the surface, respectively". Please provide literature here. 12) Reference 14: volume number (32) is missing. In addition, the article number is 2003297 rather than e2003297. 13) Methods: please change "1 mL o-tolidine indicator solution" to "1 mL of o-tolidine indicator solution"; change "20 mg catalyst decorated with CoOx cocatalyst" to "20 mg of catalyst decorated with CoOx cocatalyst"; change "0.01 mol•L-1" to "0.01 mol L-1". Please correct similar errors throughout the manuscript. 14) Reference 17: article number is 1907976 rather than e1907976. 15) Reference 23: volume number (6) is missing. 16) It would be better to compare the photocatalytic performance of the as-synthesized photocatalysts in this study with that reported in the literature to justify that this study significantly advances the development in this field and thus merits publication in Nature Communications.

Dear Editor & Reviewers,
We would like to thank you for the constructive comments and suggestions, which are very helpful for us to improve our work. We have revised the manuscript carefully, and the followings are the point-to-point responses and list of modifications.

Reviewer 1
Zou and co-workers report the preparation of chiral ZnO nanocrystals that are employed in photocatalytic O2 production. Combined experimental techniques are utilized to probe the chirality-induced spin-selectivity in O2 production. The authors have described the work well, with sincere measurement efforts. However, considering the high impact of Nat. Commun. The present work lacks novelty which does not well fit the journal's high demand.

Comment 1:
A similar kind of work with the same chiral material has already been reported with ZnO, but photocatalytic oxidation was not performed; however, several researchers have already explored the spin-filtering effect through chiral materials by Ron groups and others. Thus, I don't think that the present work has sufficient novelty or newness. As can be seen, most of the previous studies were limited to the use of chiral macromolecules to modify the surface of traditional photocatalysts. Some problems may be overlooked in those systems, such as instability, additional conductivity barrier and limited active sites upon the introduction of organic macromolecules. The direct construction of chiral oxides and usage in photocatalysis should be more attractive.
Although chiral ZnO has been prepared and multiple optical activities such as high circularly polarized luminescence and Raman optical activity have been demonstrated in the literature, the CISS effect in this material and the specific mechanism in photocatalysis has not been considered. In this work, we for the first time report how chiral metal oxides improve the carrier transport and enhance the surface redox reaction. The result demonstrates the promising potential of chiral control in photocatalysis, and the chiral metal oxide is expected to represent a new direction of photocatalyst and attracts wide interests and follow-up study. We believe our wok is a big progress in photocatalysis.
Modification 1: 1. The introduction was revised as follows.
A more universal approach to produce spin polarized carriers is using spin filters, like chiral structures, like biomolecules (proteins, oligopeptides DNA and so on) assemblies 31-35 and especially chiral inorganic materials 36, 37 , termed as chiral-induced spin selectivity (CISS) effect 38-40 . Some exciting experimental results involving the CISS effect in chiral molecules have been reported, such as spin-selective transport, long-range spin electron transport, chiro-optical response and charge polarization, enantiospecific adsorption and so on 38 . But it is still difficult to establish a quantitative relationship between optical response and the magnitude of CISS effect. In CISS effect, as the electron's velocity direction changes, the spin direction is also altered, which is equal to an effective magnetic field generated by the joint action of chiral structure and electric dipole moments. Subsequently it can control the electron's spin direction and breaks the degeneracy of spin energy level in chiral structure. The effective magnetic field can be estimated by the motion law of charge in magnetic field. Suppose that an electron of mass m moves around in a chiral molecule with helix radius r, then r=mν/e|Beff |, where e stands for electron charge and ν is for electron velocity. This can significantly affect the spin-dependent electron transfer during the catalysis.
Recently, the chiral systems have been constructed by assembly of chiral organic molecules on semiconductor like TiO2 31-33 and Fe3O4 35 photoelectrodes, which can accelerate the photocatalytic oxygen evolution from water. However, the systems involving chiral macromolecules may bring some problems, such as the instability when exposing to light and electric field, high conductivity barrier and partially shielded active sites 39 . The direct construction of chiral oxides and usage in photocatalysis should be more attractive. However, there are very less reports as to chiral oxides 41 , and the effects of the nano-scale chiral structure on the charge transfer and oxygen evolution reaction in photocatalysis has not been considered.

Comment 2:
In Fig 2d, compared to L-ZnO, why in D-ZnO current drastically increase at the low potential in the up-tip direction? Though spin-filtering is already well-studied in chiral materials, authors must briefly illustrate the mechanism in their systems.

Response 2:
Thanks for the valuable comment. The drastic current increase for D-ZnO at the low potential in the up-tip direction should be caused by the too narrow area scan of the surface of D-ZnO film, and these local structures may show relatively higher conductivity. To increase the testing accuracy, we re-tested the data for D-ZnO by increasing the selected testing points and area of the sample, and update the data in the revised manuscript. As shown in revised Figure 2d, the spin-dependent current increase for D-ZnO (D-Tipup) is similar to L-ZnO (L-Tipdown). Moreover, the original data were also added in Figure S17 (Supporting Information). We also introduce the mechanism briefly in the manuscript.   (d-f) The current as a function of the applied voltage I-V curves of D-ZnO with the tips magnetized in the nonmagnetized (black), up (blue) and down (purple).

Comment 3:
If chiral ZnO shows enhanced electrocatalytic water oxidation activity compared to achiral ZnO, why is more potential required for water oxidation in chiral ZnO, as shown in Fig 3a? Response 3: Thanks for the very valuable comment. Onset potential and photocurrent are two fundamental parameters for evaluating the performance of photoelectrode. The potential at which OER begins under light is defined as the onset potential (Eonset).
Meanwhile, the photocurrent density at 1.23 VRHE is the most important benchmark for photoelectrochemical performance, since it is the theoretical potential where the oxygen evolution reaction can occur. In the original Figure 3a, the photocurrent density of achiral DL-ZnO photoanode at 1.23 V vs. RHE is 0.21 mA•cm -2 , which is increased significantly to 0.43 and 0.40 mA•cm -2 for L-and D-ZnO with chiral structures, respectively. This indicates the chiral ZnO has much higher catalytic activity. However, the onset potential of chiral ZnO is slightly higher than the achiral one, which is unnormal. To double check this, we re-tested the photoelectrochemical performances of 10 batches of L-, D-, and DL-ZnO samples as shown in Figure   S18a-c (Supporting Information), and provided the averaged data in Figure 3a and   3. Pages 10-11, the following sentences were modified.
In Figure 3a, the photocurrent density of achiral DL-ZnO photoanode at 1.23 V vs.
RHE is 0.21 mA•cm -2 , which is increased significantly to 0.43 and 0.40 mA•cm -2 for L-and D-ZnO with chiral structures, respectively. The reproducibility of PEC performance is also measured in the Figure S18, and the statistical onset potential of chiral ZnO is slightly lower than that of achiral ZnO (Inset in Figure 3a). It is worth noting that, L-and D-ZnO show high applied bias photon to current efficiency (0.175%-0.19 %) at 0.76 V vs. RHE, which is more than 1.6 times higher than that of DL-ZnO (0.11%), as shown in Figure S19 (SI). Moreover, the incident photon-to-current conversion efficiency (IPCE) values of L-and D-ZnO photoanodes reach 36% and 27% at 370 nm, which are ca. 2.2 and 1.7 times higher than DL-ZnO (16%), respectively (Figure 3b). In addition, the performance improvement is better than the reported modification strategies for ZnO-based photoanodes (Table S1, SI).

Comment 4:
The conclusion seems exactly the same as the abstract, which should be rewritten.

Response 4:
Thanks for the very valuable comment. We have re-written both the abstract and conclusion.
Modification 4: 1. The abstract was revised as follows.
The spin degree of freedom is an important and intrinsic parameter in boosting the carrier dynamics and surface reaction kinetics of photocatalysis. Here, we demonstrate that chiral structure in ZnO can induce spin selectivity effect to promote photocatalytic performance. The ZnO crystals synthesized using chiral methionine molecules as symmetry-breaking agents show chirality in atomic and bulk levels.
Magnetic circular dichroism spectroscopic and magnetic conductive-probe atomic force microscopic measurements demonstrate that chiral structure acts as spin filters and induces spin polarization in photoinduced carriers. The polarized carriers not only possess the prolonged lifetime during transfer process, but also increase the triplet active species instead of singlet byproducts during surface oxidation reaction.
Accordingly, the left-and right-hand chiral ZnO exhibit 2.0-and 1.9-times higher activity in photocatalytic O2 production and 2.5-and 2.0-times higher activities in RhB photodegradation, respectively, compared with achiral ZnO. This work provides a feasible strategy to manipulate the spin properties in metal oxides for electron-spin-related redox catalysis.

The conclusion was revised as follows.
This work modulates the spin polarization of ZnO crystals by tuning the atomic and bulk chiral structures, to boost carrier dynamics and reaction kinetics of photocatalysis. Specifically, the chiral ZnO can create effective magnetic field and serve as spin filters via CISS effect, which induces spin polarization of carriers.
Importantly, the spin-polarized L-(or D-) ZnO shows 2.4 (1.2) times longer charge carrier lifetime and ca. 5.5-fold reduced singlet byproducts than achiral ZnO. Accordingly, L-and D-ZnO samples exhibit 2.0-and 1.9-times higher activity in O2 production and 2.5-and 2.0-times higher reaction rate in RhB degradation than achiral ZnO, respectively. Therefore, the construction of inherent chiral structure in semiconductor provides an effective way to induce the spin polarization and thus provide a new perspective for improving carrier dynamics and surface reactions in photocatalysis. For this purpose, the development of universal strategy to fabricate chiral photocatalysts is one key issue for further work.

Comment 5:
English/grammar is not up to the mark.

Response & Modification 5:
Thanks for the very valuable comment. We checked the whole manuscript and carefully revised the English/grammar, with the help of native English speaker.

Comment 6:
Challenges and future scope of this field are missing.

Response 6:
Thanks for the very valuable suggestion. We have supplemented the challenges and future scope of this field in the conclusion section.

Modification 6:
The conclusion section was revised as follows: This work modulates the spin polarization of ZnO crystals by tuning the atomic and bulk chiral structures, to boost carrier dynamics and reaction kinetics of photocatalysis. Specifically, the chiral ZnO can create effective magnetic field and serve as spin filters via CISS effect, which induces spin polarization of carriers.
Importantly, the spin-polarized L-(or D-) ZnO shows 2.4 (1.2) times longer charge carrier lifetime and ca. 5.5-fold reduced singlet byproducts than achiral ZnO. Accordingly, L-and D-ZnO samples exhibit 2.0-and 1.9-times higher activity in O2 production and 2.5-and 2.0-times higher reaction rate in RhB degradation than achiral ZnO, respectively. Therefore, the construction of inherent chiral structure in semiconductor provides an effective way to induce the spin polarization and thus provide a new perspective for improving carrier dynamics and surface reactions in photocatalysis. For this purpose, the development of universal strategy to fabricate chiral photocatalysts is one key issue for further work.

Reviewer 2
The work described the preparation and characterization of chiral ZnO layers.
Beyond serving as spin filter that can be used in reactions that involve triplet oxygen, the authors found that the chirality of the film prolongs the lifetime of the electron-hole following photoexcitation. The study is very interesting and convincing.
However, two issues must be improved before this manuscript can be accepted for publication.

Comment 1:
There are no error-bars in the data presented and not estimation of the accuracy of the result is given. In addition, it is not clear how many samples were prepared and what is the variation in their properties.

Response 1:
Thanks for the very valuable suggestion. We have added detailed sample numbers for measurements and added the error bars in figures.
In Figure 3a, the photocurrent density of achiral DL-ZnO photoanode at 1.23 V vs.
RHE is 0.21 mA•cm -2 , which is increased significantly to 0.43 and 0.40 mA•cm -2 for L-and D-ZnO with chiral structures, respectively. The reproducibility of PEC performance is also measured in the Figure S18, and the statistical onset potential of chiral ZnO is slightly lower than that of achiral ZnO (Inset in Figure 3a).
In addition, at least 10 batches of samples were tested for photoanode performance and error tapes were provided in Figure 3a.
All photocatalytic experiments were repeated more than 3 times and error bars were labeled.

Comment 2:
In the introduction it is not clear what of the properties of chiral systems mentioned were really observed and what properties one can assume to exist but they were not observed so far. A clear distinguishing between the two class must be made.

Response 2:
Thanks for the very valuable suggestion. We have added to the discussion of properties of chiral systems in the introduction section.

Reviewer 3
Most researchers regulated the spin degree of freedom in transition metals directly, which depends greatly on fine-tuning the coordinated structure of metals or producing spin-polarized carriers by using spin filters. In this study, the authors fabricated the hierarchical chirality of ZnO photocatalyst as the prototype by asymmetric coordination between the chiral center of the methionine molecules and Zn 2+ ions.
The chiral structures acted as the spin selection filter and induced spin polarization during the photoinduced carrier separation. Interestingly, the photoexcited carrier lifetime was extended by suppressing the carrier recombination, and spin-dependent kinetics of water oxidation reaction and pollutants mineralization reaction was significantly promoted by inhibiting the singlet H2O2 formation, and as a result, the left-and right-hand chiral ZnO showed greatly enhanced photocatalytic activity for the O2 production and photodegradation of pollutants. Overall, it is an interesting study, and the conclusions are mostly supported by the data, which may merit publication after fully addressing the following issues.

Comment 1:
Page number is missing in the manuscript.

Response &modification 1:
Thanks for the valuable suggestion. We have added page numbers in the revised manuscript.

Response 2:
Thanks for the very valuable suggestion. We have added "respectively" after "photodegradation".
Modification 2: 1. In Abstract, the following sentences were revised.
Accordingly, the left-and right-hand chiral ZnO exhibit 2.0-and 1.9-times higher activity in photocatalytic O2 production and 2.5-and 2.0-times higher activities in RhB photodegradation, respectively, compared with achiral ZnO.

Response 3:
Thank for the very valuable suggestion. We have added these important references in the introduction to enhance the research background.

Comment 4:
The authors claimed that chiral ZnO was fabricated by adopting the amino acid-induced self-assembly method reported in the literature (ref. 40). Did the authors modify the method?

Response 4:
Thanks for the very valuable suggestion. We have optimized the synthesis method for chiral ZnO. To fabricate the photoanode, we applied the fluorine-doped tin oxide (FTO) glass as substrate to increase the conductivity. Moreover, we systematically optimized the synthetic parameters (i.e., reagent dosage, reaction time, etc.) to improve the light adsorption of ZnO films and photoanode preformance.

Modification 4:
1. Page 15, Lines 4-6, the following sentences were revised. Chiral ZnO samples were fabricated via amino acid-induced self-assembly method according to the previous literature 42 but with optimized the parameters to get well ZnO photoanodes. Figure S2, caption: "is precursor" should be replaced by "are the precursors".

Response 5:
Thanks for the very valuable suggestion. We have replaced "is precursor" to "are the precursors".

Response 7:
Thanks for the very valuable suggestion. We uniformed the scale to 200 nm in revised Figure S8b-b2.

Response 8-10:
Thanks for the very valuable suggestions. We have provided the pore size distribution of samples in Figure S9b. As shown in Figure S9b, L-ZnO, D-ZnO and DL-ZnO all possess the similar mesopores with size of 2.8 nm, and they also have mesopores with size of 40.1, 29.8 and 18.3 nm, respectively. The total pore volumes are 0.18, 0.20, and 0.18 cm 3 /g, respectively. Although the presence of mesoporous structure is beneficial to enhance light scattering/reflection and provide active sites, the difference in between the three samples is very small, so it should be not the key factor to affect the PEC performance. 1. Figure S9 was supplemented as follows. Although the presence of mesoporous structure is beneficial to enhance light scattering/reflection and provide active sites, the photocatalytic activity difference between the three samples due only to the surface area and porosity structure will be relatively small.

3.
The following references were supplemented in Supporting Information.

Response 15:
Thanks for the very valuable suggestion. We have added the volume numbers (6) to reference 23.

Comment 16:
It would be better to compare the photocatalytic performance of the as-synthesized photocatalysts in this study with that reported in the literature to justify that this study significantly advances the development in this field and thus merits publication.

Response 16:
Thanks for the very valuable suggestion. We compared the photocurrent of chiral ZnO with those of ZnO-based photoanodes in previous literatures. And the chiral ZnO shows much better performance, confirming the advance of our strategy to improve the photocatalytic performance.
Modification 16: 1. Table S1 has been added in Supporting Information.